This invention relates generally to swaged collar fasteners and more particularly to pull type swaged collar fasteners which have breakoff pintails thereon to provide a gripping means for the swaging tool.
The fasteners to which this invention relates include a headed pin with locking grooves disposed thereon and a collar having a cylindrical wall. When such fasteners are utilized to secure a composite assembly of workpieces, the pin is disposed into aligned holes through the workpieces to be secured in the composite assembly and the collar is disposed about the pin over the locking grooves thereof. Then the cylindrical wall of the collar is radially compressed into the locking grooves to affix the collar on the pin. Various tools are available for this collar wall compression operation with the most common being a swaging tool which pulls the pin in one direction while forcing a swage anvil over the collar in the other direction to forceably clinch the workpieces of the composite assembly together while affixing the collar to the pin. The pull is applied to the pin through a pintail disposed on one end thereof, which is broken off and discarded as scrap, after the collar has been affixed to the pin.
One problem encountered with swaged collar fasteners is that success of the collar wall compression operation in filling the pin locking grooves with material from the collar, cannot be guaranteed by design of the fastener. This operation depends on so many variables, such as the relative hardness and shear strengths of the materials in the pin and collar, the dimensional tolerances of both the pin and collar, the elasticity of the pin and etc., that previous attempts to predetermine and assure such success have all generally failed. Such failure is attested to by the "overpacking" approach that is taken to the problem in U.S. Pat. No. 4,472,096 and the head dishing approach which is utilized to provide "physical evidence" that a composite assembly is being secured by a swaged collar fastener in U.S. Pat. No. 4,324,518. When success of the collar wall compression operation is assured without regard for the peak loads which are attendant to the "overpacking" approach, damage often occurs to the fastener pin and/or the workpieces being assembled thereby, to frustrate such success. This is particularly true when the aligned holes for the fastener pin are disposed through thin and/or nonmetallic materials, such as woven graphite-epoxy composites.
A chamfer has been utilized at one end of the collar bore to provide very limited stress relief for the collar wall compression operation, as disclosed in U.S. Pat. No. 3,215,024. However, the volume of such chamfers is too small to receive the amount of material flow that is necessary to cope with the many variables affecting the collar wall compression operation, which were mentioned previously. Furthermore, stress relief distribution over the collar during that operation is impossible with the chamfer approach. Therefore, widely varying peak loads are encountered therewith at different locations on the collar.
Another problem encountered with swaged collar fasteners is that vibrations passing through the composite assembly can cause the fastener collar to turn on the fastener pin. Where annular locking grooves are utilized on the fastener pin, such collar turning presents little adverse affect in low vibration environments for some time, until such locking grooves and/or the collar material therebetween starts to wear away and the collar loosens to eliminate the clinching force that tightly retains the workpieces in the composite assembly. Of course, in a high vibration environment such wear would be greatly accelerated and the collar loosening would occur in a relatively short time. Where helical locking grooves are utilized on the pin, the collar will only turn in one direction but any such turning will immediately eliminate the clinching force retaining the workpieces in the composite assembly because the collar will immediately loosen on the pin.
When the pin is fabricated from expensive materials, such as titanium alloys, the use of a pintail thereon is always viewed with some apprehension because it serves no useful purpose after the collar is affixed to the pin and in most composite assemblies is removed and discarded as scrap. As is presently known, what had been accepted in the past as the required length of a pintail, has now been reduced and U.S. Pat. No. 4,221,152 provides one solution to the land hangup problem encountered with a conventional swaging tool due to that reduced pintail length. However, other solutions to this problem and other approaches to reducing the volume of material in the pintail would certainly gain immediate adoption by the fastener industry.